I've had my chances to travel from Tokyo to Slovakia and everywhere in between. I absolutely love Paris. I worked in the eastern part of France in the country where the people we friendly and the food had a Germanic flavor to it. I spent a lot of time along the Rhine river visiting castles too. My favorite vacation was on a narrowboat (canal boat) in England. So darn relaxing cruising along at 3 knots and stopping at every pub along the way. Everyone was super friendly.
Northern England is very nice with castles and very friendly people. Germany is also great with great food and, of course, beer AND fun loving people.
I had a chance to visit long lost relatives in Slovakia a few years ago. The country is absolutely breath taking from farm land to snow covered mountains to caves to a multitude of castles and wooden churches. Great and cheap food and pastries. I'd go back in a minute given the opportunity.
Lived in Hawaii but still enjoy going back. Lived in Guam (another story) and visited Palau (great snorkeling) and Yap. Those would be interesting now that they have been built up and modernized. When I was there, there were still Jap planes along the runway!
Austria is also a beautiful country.
It really depends what they are interested in. Scenic beauty, history, or architecture.
Places I haven't been but would love to see are New Zealand, as some else said, and Australia. Pretty much everything else has been over run and ruined by Muslims, including Spain and London.